1. Field of the Invention
This invention relates to an ink jet printing system and more particularly to a thermal drop-on-demand ink jet printing system.
2. Description of the Prior Art
A thermal drop-on-demand ink jet printing system is known in which a heater is selectively energized to form a "bubble" in the adjacent ink. The rapid growth of the bubble causes an ink drop to be ejected from a nearby nozzle. Printing is accomplished by energizing the heater each time a drop is required at that nozzle position to produce the desired printed image.
The formation of the vapor and gas "bubble" on a small heater is normally not well-controlled in terms of nucleation sites and timing. U.S. Pat. No. 4,366,548 to Matsumoto discloses a thermal drop-on-demand ink jet printing system in which the entire heater is covered by a protective layer, and the surface of the protective layer, to which the ink is exposed, is roughened. The roughness of the protective layer is described as an aid to the nucleation process in bubble formation.
U.S. Pat. No. 4,339,762 to Shirato et al relates to thermal drop-on-demand ink jet printing system in which the heat generating element is non-uniform in either thickness and/or width so that the size of the ejected drop can be controlled by controlling the amplitude of the drive signal applied to the heat generating element.
U.S. Pat. No. 4,514,741 to Meyer shows a thermal drop-on-demand ink jet printer in which the heater element comprises a resistive region having a conductive region at its center. The conductive region effectively electrically shorts the underlying area of the heater element which produces a cold spot at the center of the heater element and enables the production of a toroidally shaped bubble.
No prior art is known in which a heat delay means is utilized to cover a predetermined part of the heating means to produce a controlled bubble growth and collapse so that the print head operation is enhanced by utilizing the inertial effects of a controlled bubble motion.